WaitManager

[Robert O'Callahan]

I just landed https://github.com/rr-debugger/rr/pull/3378 which introduces WaitManager, a wrapper around waitpid/waitid syscalls. The basic idea is to be able to wait for stops from any task and stash them in the WaitManager if it turns out that we don't want to process those stop(s) right now. This lets the Scheduler wait for stops from all tasks at once instead of doing one waitpid syscall per task, which doesn't scale very well for applications with large numbers of threads.

From now on, instead of calling waitpid/waitid directly, we should use WaitManager APIs to ensure that if WaitManager has cached a stop, that will be reported if applicable.

On another note, the various flags tracking task state in Task and RecordTask are really getting out of control and need to be cleaned up. I'm still thinking about how that might be done.

Rob

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